The invention relates to an RF-tuning device comprising a switching mixer stage and a tuning oscillator coupled thereto.
Such an RF-tuning device is used in the TV-UHF/VHF tuning unit UV 411 described in the "Philips Data Handbook", C2, No. 12, 1982 "TV-tuners, Video-Modulators and SAW-filters", pages 141-143, published by Philips Electronic Components and Materials Division.
In the prior art RF-tuning device, additive mixing of a desired TV-VHF receiving signal with an oscillator signal is effected at the base-emitter junction of the mixing transistor, the mixing transistor being switched by the oscillator signal. Because of the signal gains in the signal processing operations effected subsequent to the RF-tuning device before the desired TV-signal is suitable for display, the inherent noise contribution or the noise factor, as the case may be, of the RF-tuning device and more specifically the noise factor of the mixer stage comprised therein, must be kept as low as possible.
It is known that the noise factor of the mixer stage can be reduced by increasing the amplitude of the oscillator signal. However, increasing the oscillator signal also results in an increase of oscillator radiation. As the requirements as regards the maximum permissible oscillator radiation are very strict in some countries, and moreover a large, stable and tunable oscillator signal cannot be realized with simple means, this manner of reducing noise can, in practice, only be used to a limited extent.